1. Field of the Invention
This invention relates to a repair process for fibre reinforced structures, especially but not exclusively graphite or composite graphite structures. It finds particular application to sports equipment such as tennis, squash and badminton racquets.
2. Description of the Prior Art
Equipment which must be light but strong is commonly fibre reinforced, and may be made at least partially from graphite or composite graphite materials. This is often the case with sports racquets such as tennis racquets. The shaft, handle and strung head portion of a racquet frame may each or all be constructed out of these materials and the usual associated structure comprises a hollow shell of laminated carbon (graphite) fibres set in resin, the hollow centre being either empty or filled with a filling material such as plastics, vibration-dampening foam, timber, cork, or alloy extrusions. Sometimes, particularly in the shaft, the shell of laminated fibre may have more than one internal cavity, the shell cross section then appearing for instance as a figure-of-eight.
In the event of damage to a graphite or composite graphite structure, it is necessary to effect a repair which will at least approximately equal the strength of the unbroken structure.
According to a known technique for repairing sports racquets, the following steps are applied:
(i) if the repair is to a strung area of the racquet, the strings are removed, together with any protective plastic grommet strip;
(ii) a temporary bond is made across the fracture with cyanoacrylate glue and microsphere ballons as a filler and a check is made that the frame is straight at the fracture;
(iii) a section of the graphite material shell is cut out to form a window exposing the hollow centre and extending across and to either side of the fracture;
(iv) any foam core or other lining or filling material is carefully removed from the cavity exposed at the window;
(v) the cavity is blown free of dust, epoxy resin is painted onto the walls of the cavity and unidirectional Kevlar rovings are wet out and placed along the walls of the cavity. If the fracture is in a strung area, it is checked that no Kevlar rovings are laid across a wall of the cavity through which a string hole will have to be redrilled subsequently;
(vi) fibre glass rovings, such as those used in a chopper gun for boat construction, are inlaid until the cavity at the window is full and the fracture is left to cure overnight;
(vii) the fibreglass rovings are filed to match the outline of the original frame and paintwork is removed as far as perhaps 10-12 mm to either side of the window;
(viii) "2 oz" glass cloth, saturated with resin, is wrapped around the frame across the fracture so as to cover and provide a splint for the internal inlay, and the frame is again left to cure overnight;
(ix) the frame is again filed at the fracture, the glass cloth overlay being tapered to meet the existing frame, the frame being first rough filed then smooth filed and finally sanded;
(x) any string holes are redrilled, the frame is repainted, the grommet strip replaced and the racquet restrung.
Repair methods as described above are very time-consuming. Altogether, the process takes about three days and involves significant down time when the racquet is merely left to cure.
Also, there is a tendency for the fibreglass rovings in the cavity to swell and contract unpredictably during curing. When this occurs after the glass cloth overlay is in place, a depression or a bulge can be produced in the overlay which cannot be put right since the problem stems from underneath, in the inaccessible fibreglass rovings. The result is that quite often there is an unpredictably misshapen repair section.